[Task]: Rename AddonApprovalsCounter model

[eviljeff]

Description

follow-up to mozilla/addons-server#24308

The model tracks many things to do with approvals, but the counter part is the least important. Lets rename the model to something more accurately (and less awkwardly) named,

Acceptance Criteria

• model not named AddonApprovalsCounter
• same database table name - we don't need a complicated migration process and broken redash queries.

┆Issue is synchronized with this Jira Task

[diox]

Maybe worth looking into getting rid of it and use AddonReviewerFlags instead